Electrical audio signal processing systems and devices – Noise or distortion suppression
Patent
1995-06-12
1998-11-10
Kuntz, Curtis A.
Electrical audio signal processing systems and devices
Noise or distortion suppression
381 92, 379406, 379410, 379411, H04B 1500
Patent
active
058356070
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a mobile radio transceiver with a hands-free facility, which hands-free facility comprises combining means for combining at least two acoustic input signals, and an adaptive filter for filtering the combined output signal of the combining means.
The invention further relates to a hands-free facility which comprises combining means for combining at least two acoustic input signals, and an adaptive filter for filtering the combined output signal of the combining means.
2. Discussion of the Related Art
A hands-free facility with four microphones is known, for example, from "A MICROPHONE ARRAY WITH ADAPTIVE POSTFILTERING FOR NOISE REDUCTION IN REVERBERANT ROOM" by Rainer Zelinski, ICASSP 88, pp. 2578 to 2581. In a first processing step the directivity gain of the two-dimensionally arranged microphones is used for noise reduction. In a second processing step the microphone signals are further processed by an adaptive Wiener filter which estimates the desired speech signal. For computing the parameters of the Wiener filter the autocorrelation function and the crosscorrelation function of the input signals are measured.
SUMMARY OF THE INVENTION
It is an object of the invention to improve the speech quality in a mobile radio transceiver and in a hands-free facility of the type defined in the opening paragraph.
In a mobile radio transceiver as well as in a hands-free facility of the type defined in the opening paragraph this object is achieved in that a high-pass filter is provided for filtering the acoustic input signals.
The invention is based on the recognition that various undesired side effects occur as a result of the large noise component in the microphone signals i.e., in the acoustic input signals in a motor car when the parameters for the adaptive filter are computed, which side effects lead to a degradation of the speech quality of the output signal. For processing the acoustic analog input signals coming from a group of microphones arranged in an array, these input signals are first converted, for example, into digital signals by analog-to-digital converters. The combining means following in the circuit are used for combining the digital input signals thus obtained to an output signal that contains the least possible interference. The adaptive filter arranged downstream of the combining means is used for reducing disturbing noise which often occurs with a hands-free facility arranged in a motor car, for example, as a result of engine noise. Especially the use of high-grade microphones may call forth a degradation of the output signal of the hands-free facility.
In accordance with a principle of the invention the high-pass filtering of the acoustic input signals surprisingly simply achieves a reduction of these effects. For example, a 4.sup.th order digital infinite impulse response (IIR) filter can be used as a high-pass filter. Subjecting the input signals to such a high-pass filtering provides that the poorer signal processing relating to the low-frequency portions of the input signals in the hands-free facility may be omitted, so that on the whole an improvement of the speech quality of the output signal of the hands-free facility of the mobile radio transceiver is obtained. This achieves that the relatively high signal energy of the low-frequency range, which energy is mainly formed by interference signals, is filtered out and thus an improvement of the speech quality of the hands-free facility and thus of the mobile radio transceiver is effected.
An improved speech quality of the hands-free facility of the mobile radio transceiver may be achieved when the limit frequency of the high-pass filter lies in the range from 200to 400 Hz. Especially with a 300 Hz limit frequency of the high-pass filter a considerable improvement of the output signal could be realised, as frequencies lying below 300 Hz do not play a role anyway in telephone speech relating to a mobile radio transceiver.
A further improvement of the speech qua
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A Microphone Array with Adaptive Postfiltering for Noise Reduction in Reverberant Room by Rainer Zelinski, ICASSP 88, pp. 2578 to 3581.
Janse Kees
Kellermann Walter
Knibbler Charles
Martin Rainer
Biren Steven R.
Kuntz Curtis A.
Nguyen Duc
U.S. Philips Corporation
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